Control Strategy for Smooth Switching of Substation Level Microgrid Operation Mode under Weak Grid

In allusion to substation level microgrid with master-slave control structure under weak grid, the control modes of energy storage and diesel generating sets under grid-connected condition and island operation condition were analyzed, and the input circuit of the controller for diesel generating sets had been improved to make it suitable for microgrid system with multi- grid-connecting positions. Firstly, a smooth switching over strategy for the substation level microgrid, in which adaptive energy storage or diesel generating sets were taken as the main power source, was proposed. Secondly, during the switching over process from grid-connected condition to island operation condition, based on the respective power output characteristics of energy storage and diesel generating sets the optimized and coordinated distribution of the tie-line power was performed. Thirdly, utilizing the interaction of the step-size control of power via voltage blocking with the regulation characteristics of line voltage regulator, the fluctuation of system voltage and frequency were effectively suppressed. Finally, the combination of self-synchronizing control with multiple synchronization control was adopted to improve both efficiency and success rate of switching over from island operation condition to grid-connected condition. The effectiveness of the proposed control strategy is verified by the results of on-site test for practical substation level microgrid project.